Devices for inductive transmission of electrical energy are used, among other purposes, for charging a rechargeable battery installed in an electric vehicle. During energy transmission, a magnetic field of high field intensity and flux density is generated between a stationary primary coil and a secondary coil on the vehicle side. This is necessary in order to induce, in the secondary coil, a sufficiently high current for the desired transmission of power.
If objects made of metal materials are introduced into the region of such a field, eddy currents are induced, which lead to heating, as a function of material, duration of exposure and field intensity level. If corresponding conditions are present, such an object can reach a temperature that can lead to damage, for example, melting, in plastic surfaces, or to hazards to people. The latter occurs particularly if the secondary side has been removed and the heated metal objects are freely accessible so that come in contact with people. For a largely automatic operation of inductive charging stations for electric vehicles, particularly when such charging stations are used in publicly accessible areas, effective security measures are needed to prevent damage and hazards for people due to heated metal foreign bodies.
From DE 10 2009 033 237 A1, a device for the inductive transmission of electrical energy is known, in which, for the detection of a conductive foreign body via the primary coil, a plurality of measuring inductances are arranged, each connected to an impedance measurement device and to a common evaluation device. In the process, a separate impedance measurement device can be associated with each measuring inductance, or several measuring inductances can be connected via an analog multiplexer to a common impedance measurement device. Moreover, the measuring inductances can also be connected to one another to form groups, by means of which the resulting impedance of the entire group is then measured. The publication leaves open the question to which of the mentioned possibilities to give preference.